Janoris Jenkins would have done the Giants a huge favor by getting himself suspended for Sunday’s game against the 49ers — because he was as big a reason as anyone for the Giants’ 31-21 loss to the 49ers with his poor execution and worse effort.

Jenkins, who was suspended for last week’s game after inexplicably failing to return from the bye week on time, has become a symbol of how far the Giants’ defense has fallen from a year ago.

Unlike the rest of his teammates, who were accountable and spoke to reporters after the game, Jenkins blew off reporters in the locker room.

“I’ll be back,” the Giants cornerback, who was signed to a five-year, $62.5 million contract with $28.8 million guaranteed, said as he blew past reporters and never returned.

If only he showed that much burst on the field, instead of watching 49ers receiver Marquise Goodwin race past him on an 83-yard touchdown catch-and-run to give San Francisco a 10-6 lead in the second quarter.

If only he showed that kind of determination as he whiffed on an arm-tackle effort as 49ers tight end Garrett Celek was heading to the end zone on a 47-yard touchdown catch-and-run that gave San Francisco a 17-13 lead it did not relinquish.

The 49ers entered the game having scored a total of 30 points in their previous three games — all losses. Rookie quarterback C.J. Beathard, who entered the game with two touchdown passes this season, matched that total Sunday against the Giants. The 49ers entered the game converting 31.3 percent on third downs and were 8-of-12 Sunday.

“You don’t want to come out here and put on a show like this against a team that didn’t have any wins,” Giants linebacker Jonathan Casillas said. “That’s pretty tough to swallow. We’re not a good team right now.”

Casillas was victimized in coverage by Celek on the TD.

“On the play that I gave up the touchdown, I was running but I was a little tired, to be honest with you,’’ Casillas said. “I play my butt off on every play and I just wasn’t good on that play. We can’t keep putting up games like this. We only have one win and it’s going to stay that way until we can figure out a way to slow these teams down.’’

Defensive end Olivier Vernon, who had an interception in the second half, blamed the loss on the defense — the same defense that was rebuilt last year with more than $200 million in contracts.

“We made some costly mistakes, and that’s on us,” he said. “There’s no excuse for that. It’s on us as a defense to get it right. That’s been a theme all year — the big plays get us. We can’t win no games like that, especially as a defense that takes pride in stopping the run and stopping the pass. And we didn’t do any of that today.”